The present invention generally relates to torsion bars usable in energy absorbing seat belt retractors.
The classic type of seat belt retractor comprises a frame with a spool rotationally mounted upon the frame. The spool will typically include one or more lock wheels each having a plurality of teeth which are engaged and locked by a corresponding lock pawl. The lock pawl or lock dog is rotationally mounted to the frame and movable from a disengaged position to an engaged position with a tooth of the lock wheel. In this type of retractor once the spool is locked, further rotation of the spool is prohibited. One skilled in the art will appreciate that all forward motion of the occupant will not be stopped in this type of retractor because as the occupant loads the locked retractor, the seat belt is stressed and stretches and the seat belt slips over itself (the so called film spool effect).
However, with an energy absorbing retractor, the spool and its associated mechanisms are permitted to rotate and the seat belt is controllably permitted to protract in response to the load imparted to the seat belt by the occupant. The forward motion of the occupant is restricted by a reaction force or torque generated within the retractor and modified by the stretching seat belt. In this way the protraction of the seat belt and the forward motion of the occupant are controlled. Energy absorbing seat belt retractors often employ a deformable member such as a crushable bushing or a torsion bar. In either case, the bushing is crushed or the torsion bar twisted beyond its elastic limit into its plastic range or zone of operation to generate the desired (theoretically constant) reaction torque which acts against the torque transferred to the retractor spool via the forces imparted to the seat belt by the moving occupant.
The goal of an energy absorbing retractor is to generate a generally constant reaction force to oppose the forward motion of the occupant and to be able to generate this reaction force during the accident event, that is, during the entire time that the seat belt is loaded by the occupant. In theory this can be achieved by utilizing a crush bushing or torsion bar that always operates in its constant plastic zone.
In a torsion bar, seat belt retractor, one end of the torsion bar is fixedly attached to a lock wheel and the other end is fixed to the retractor spool. During an accident the lock wheel is prevented from rotating by interposing a lock dog or lock pawl within the teeth of the lock wheel. As the seat belt is loaded by the occupant, the spool will tend to rotate in opposition to the reaction torque generated within the torsion bar, as the torsion bar is twisted. The generated reaction torque depends upon the amount that the torsion bar is rotated or twisted as well as upon the physical characteristics of the torsion bar.
More specifically, the reaction torque generated by a torsion bar will vary depending upon whether the torsion bar is in its elastic, transition or plastic zones or ranges. As mentioned, in an ideal torsion bar, the elastic range is characterized by a steep (preferably infinitely steep slope or deflection curve) and the plastic range is characterized by a perfectly constant torque deflection region having a sharp transition from the elastic region. In this ideal torsion bar and corresponding seat belt retractor, once a first end of the torsion bar is locked and the spool loaded, the torsion bar will immediately make a transition from its elastic range (see curve 100 of FIG. 1) into the plastic range of operation such that a constant reaction force is generated by the retractor as the seat belt is protracted.
Prior art torsion bars have been made using a number of different manufacturing methods. In one method, an over-sized metal bar is machined to reduce its diameter to a desired dimension. Subsequently, end formations are formed on the machined bar such as by cold rolling. The machining of the bar may produce stress risers which are typically non-uniform and the cold rolling of the machined bar, it is believed, reorients the grain structure of the metal in an undesirable manner. To make the stress distribution within the torsion bar more uniform, an annealing step is often used, which adds to the cost of the final product. However, this type of torsion bar does not achieve the objects of the present invention as it displays the characteristic torque deflection curve similar to that shown in curve 102 of FIG. 1 having an elastic zone, an extended elastic/plastic transition zone and a plastic zone. In another method of manufacture the torsion bar is made using a cold-formed process in which a metal bar or wire (large diameter), has a diameter less than the desired dimension. The smaller than desired diameter bar is expanded into a bar having the desired larger diameter. This type of bar has been tested and it displays or shows a characteristic torque deflection curve similar to that of curve 102 of FIG. 1. The prior art has also suggested a method of making a torsion bar having a shortened or abrupt elastic/plastic transition zone. In this method a pre-machined or pre-formed torsion bar is work hardened (by being pre-torqued or twisted beyond its yield torque level) prior to installation within a seat belt retractor. One potential deficiency of this technique is that the pre-twisting reduces the useful range through which the torsion bar can be additionally twisted, during an accident, once installed within a retractor.
I have previously proposed another methodology of making a torsion bar for use within a seat belt retractor. The torsion bar was formed of a ductile, elongated body, located between the end formations and formed by pre-stressing bar stock by extruding an oversized metal bar into a bar of a reduced diameter with its grain structure in the vicinity of a center of the bar aligned in a longitudinal direction. The end formations of the torsion bar were formed by a cold heading process. In this process the cold headed bar was not annealed. In this process, the cold heading did not disturb the longitudinal direction of the grain structure in the central portion of the torsion bar.
I have found that if the bar stock is first extruded, with the formations cold headed as I had first proposed but subsequently if the torsion bar is annealed but not at a temperature or duration which will not cause the grain structure to increase and the torsion bar is pre-twisted, excellent results can be expected.
It is an object of the present invention to provide a torsion bar which displays an abrupt transition from its elastic zone to its plastic zone. A further object of the invention is to provide an energy absorbing seat belt retractor which uses the above type of torsion bar.
Accordingly, the invention comprises: a torsion bar having a circular cross section where the torsion bar is formed as a cold headed extruded part. Preferably, the grain structure of the metal is aligned along the axis (axial direction) of the bar. If the grain structure diverges from the axial direction near an end formation it does so smoothly. Subsequent to being cold headed, the bar is annealed and thereafter twisted to generate a determinable level of work hardening. For the disclosed bar the amount of twist is about 180 degrees (0.5 twist).
Additionally, the invention comprises a seat belt retractor having: a frame; wherein the torsion bar is rotationally supported relative to the frame and wherein the torsion bar is capable of generating a determinable reaction torque as it is twisted (with one end thereof fixed). The torsion bar is characterized by an elastic deformation zone and a sharp onset into a plastic deformation zone. The retractor also includes a spool operatively connected to rotate with the torsion bar; lock means, adaptable during a vehicle accident and operatively linked to the torsion bar for preventing one side of the torsion bar from rotating while permitting the other side and the spool to rotate once loaded by the occupant.